Processing used motor oil has been a problem in search of solution for over 50 years. It is a problem both in size and technology. In the USA, over 1 billion gallons of used motor oil is “produced”. Little of it is recycled or used effectively and much is improperly dumped. Re-refining is a problem because the very additives which make modern lubricating oils stick to metal surfaces in an engine greatly complicate recovery of the lubricant boiling range hydrocarbons, at least recovery using commercially viable technology. The state of the art of producing, collecting and re-refining of used motor oil and other industrial oils will be reviewed.
Automotive and many industrial lubricating oils are usually formulated from paraffin based petroleum distillate oils or from synthetic base lubricating oils. Lubricating oils are combined with additives such as soaps, extreme pressure (E.P.) agents, viscosity index (V.I.) improver, antifoamants, rust inhibitors, anti-wear agents, antioxidants, and polymeric dispersants to produce an engine lubricating oil of SAE 5 to SAE 60 viscosity.
After use, this oil is collected from truck and bus fleets, automobile service stations, and municipal recycling centers for reclaiming. This collected oil contains organo-metallic additives such as zinc dialkylthiophosphate from the original lubricating oil formulation, sludge formed in the engine, and water. The used oil may also contain contaminants such as waste grease, brake fluid, transmission oil, transformer oil, railroad lubricant, crude oil, antifreeze, dry cleaning fluid, degreasing solvents such as trichloroethylene, edible fats and oils, mineral acids, soot, earth and waste of unknown origin.
Reclaiming of waste oil is largely carried out by small processors using various processes tailored to the available waste oil, product demands, and local environmental considerations. Such processes at a minimum include partial de-watering and coarse filtering. Some more sophisticated processors may practice chemical demetallizing or distillation. The presence of organo-metallics in waste oils such as zinc dialkylthiophosphate results in decomposition of the zinc dialkyldithiophospnate to form a carbonaceous layer rich in zinc and often other metals such as calcium, magnesium and other metals present as additives and thus difficult if not impossible to process. The carbonaceous layer containing the various metals forms rapidly on heated surfaces and can develop to a thickness of more than 1 mm in 24 hours. This layer not only reduces the heat transfer coefficient of tubular heaters rapidly, it also results in substantial or total occlusion of these tubes within a few days.
Successful reclaiming processes require the reduction of the organo-metallics (or ash) content to a level at which the hot oil does not foul heated surfaces. Such reduction can be carried out by chemical processes which include reacting cation phosphate or cation sulfate with the chemically bonded metal to form metallic phosphate or metallic sulfate. U.S. Pat. No. 4,432,865 to Norman, the contents of which are incorporated herein by reference, discloses contacting used motor oil with polyfunctional mineral acid and polyhydroxy compound to react with undesired contaminants to form easily removable reaction products. These chemical processes suffer from attendant disposal problems depending on the metal by-products formed.
Ash content can also be reduced by heating the used lubricating oil (ULO) to decompose the organo-metallic additives. Direct contact heating of ULO with a recycled bottoms fraction was disclosed in U.S. Pat. No. 5,447,628 to Harrison, et al., the contents of which are incorporated herein by reference. The ULO was added to a lower section of a vacuum column with an enlarged bottom section. There was enough volumetric capacity below the first tray of the column to provide “a residence time of 10 to 120 minutes.” The EXAMPLE reported that a residence time of 45 minutes and a relatively constant temperature of 660° F. A dehydrated ULO fraction was mixed with a recycled bottoms fraction in the rate of 1:45. The long residence time and high temperature were believed sufficient to decompose the additives in the ULO, so that a bottoms fraction from this column could be sent to a fired heater, to supply the heat needs of the process. The patentee reported that additive decomposition began at 400° F. The Figure in the patent showed that zinc compound decomposition was a function of temperature, with time temperature decomposition profiles presented for 400° F., 500° F., 750° F. and 1000° F.
UOP's Hy-Lube process, described in U.S. Pat. No. 5,244,565, U.S. Pat. No. 5,302,282, and many more patents, uses a hot circulating hydrogen rich stream as a heating medium to avoid deposition of decomposed organo-metallic compounds on heating surfaces.
The problem of fouling of heated surfaces can be ameliorated to some extent by gentler heating. Some processes, such as the fixed bed version of catalytic cracking, the Houdry process, used a molten salt bath to provide controlled, somewhat gentle heating of vaporized liquid hydrocarbon passing through tubes of catalyst immersed in the salt bath. Molten metal baths have also been used as a convenient way to heat difficult to processes substances to a control temperature, e.g., flammability of some plastics is tested by putting a flask with plastic into a bath of molten metal. Use of molten salt bath, or molten metal bath, or condensing high temperature vapor, could be used to reduce uneven heating of heat exchange surface and thereby reduce ΔT across a metal surface and perhaps slow the fouling of metal surfaces in ULO service, but the additives in the ULO would still tend to decompose on the hottest surface, which would be the heat exchanger tubes.
In U.S. Pat. No. 7,150,822 and U.S. Pat. No. 7,241,377 Malone taught use of a molten metal or molten salt bath for direct contact heating of ULO. The process effectively heats ULO without fouling the heating surface, a molten metal or salt bath, but the process requires a large and heavy molten metal vessel for processing of the oil. Start-up of such a process, or perhaps operation, may have encountered problems as the first commercial unit is believed no longer in operation.
Solvent extraction with light paraffin solvents such as propane, butane, pentane and mixtures thereof have been practiced by Interline and others. Details of the Interline Process are provided in U.S. Pat. No. 5,286,380 and U.S. Pat. No. 5,556,548. While the extraction approach seems like an elegant solution to the problem of processing ULO, the process may be relatively expensive to operate. Their quarterly report of May 15, 2002, reports that “It has become evident that demanding royalties based on production is impractical in many situations and countries. Unless and until the re-refined oil produced in a plant can be sold at prices comparable to base lubricating oils, collecting royalties based on production will be difficult. This reality was experienced in Korea, where the royalty was terminated for the first plant, and in England where the royalties were reduced and deferred until the plant becomes profitable.”
Another approach to ULO processing was direct contact heating of the ULO with steam or a non-hydrogenating gas. This solved the problem of zinc additive decomposition fouling of hot metal surfaces, by ensuring that the metal surfaces holding the ULO were always relatively cool. The hottest spot in these ULO process was the point of vapor injection. Decomposing additives had only themselves to condense upon.
A vapor injection ULO process was disclosed in U.S. Pat. No. 6,068,759 Process for Recovering Lube Oil Base Stocks from Used Motor Oil and U.S. Pat. No. 6,447,672 Continuous Plural Stage Heated Vapor Injection Process for Recovering Lube Oil Base Stocks from Used Motor Oil. The heated vapor was steam, methane, ethane, propane or mixtures. Other variations on the theme of ULO vapor injection are disclosed in U.S. Pat. No. 6,402,937 Pumped Recycle Vapor and U.S. Pat. No. 6,402,938, Vaporization of Used Motor Oil with Non-hydrogenating Recycle Vapor, which are incorporated by reference. This approach used a “working fluid” such as methanol or propane, which was heated and mixed with ULO to vaporize lube boiling range components. A lube fraction was recovered as a product and the methanol or propane working fluid either compressed, or condensed and pumped, through a heater to be recycled to heat incoming ULO.
Petroleum refiners have been trying for over half a century to devise a satisfactory way to reprocess used lube oil. No process is known which could be considered a commercial success. Despite the abundance of a potentially valuable waste material, the lubricating oil boiling range hydrocarbons trapped in the ULO, most ULO is not re-refined. The “state of the art” of used motor oil processing could be summarized as follows:
Chemical additive and extraction approaches can be used to react with or extract everything but zinc additives, but costs associated with such processes are apparently high, as evidenced by little commercial use. Additives could be extracted, but the operating costs are high.
Indirect heating in a fired heater causes rapid fouling of metal surfaces. Using milder heating, via a double boiler approach or molten metal heating medium, can minimize but not eliminate fouling on hot metal surfaces.
Direct contact heating with high pressure hydrogen may eliminate fouling but requires high capital and operating expenses.
Direct contact heating, with a recycled bottoms fraction can still suffer from heater fouling. A stream containing the non-distillable additive package and/or the decomposition products thereof is still sent through a fired heater where fouling can occur.
Direct contact heating with steam or a light hydrocarbon “working fluid” vapor is an attractive approach. When steam is the injected vapor, the process can create a water disposal problem and is thermally less efficient because the latent heat of water is lost when the steam is condensed against cooling water or air in a heat exchanger. There are also concerns about possible formation of emulsions or corrosive regions in portions of the plant, when water is condensing. When a “working fluid” is used for heating e.g., propane, the water problem is largely eliminated, but large volumes of vapor are needed to provide sufficient heat input, so costs increase to heat and recycle such vapor streams. The working fluid approach also calls for a somewhat higher capital cost, because higher pressure operation is generally needed to facilitate circulation of the large volumes of working fluid to heat the used lube oil feed.
We wanted a better approach, one which is simple and reliable and which does little or no harm to the used lube oil fraction. We define harm in the sense of thermally cracking the ULO and generating large amounts of reactive intermediate species, many of which are chloride containing.
Brute force heating, by recycling a bottoms stream, forces at least some of the additive package to end up in the bottoms and go through a fired heater, and cause fouling. The brute force approach vaporizes the lubricant boiling range components, but can easily degrade the lube components and contaminate them with significant amounts of the breakdown products of the additive package. The recovered lube boiling range components will have significant value as fuel or cracker feed blending component, but are generally not suitable for use as lubricant blending stock, at least not without a lot of expensive hydrotreating. Destructive distillation of ULO, by spraying it on top of a coker drum, decomposes the additive package and leaves it behind in the coke—but the valuable, paraffinic lubricant boiling range hydrocarbons are converted to coker naphtha or other reactive and difficult to process fractions. The lube fraction is arguably “recovered” but is no longer remotely suitable for use as a blending component.
Steam injection, for heating of ULO, would minimize thermal degradation of lubricant boiling range hydrocarbons in the ULO, but the relatively “wet” approach causes concerns about disposal of waste water, emulsion formation and/or plant corrosion. The “pumped vapor” approach, using propane or the like, eliminates most water problems, but requires a more complicated plant to recycle the hydrocarbon vapor. Large molar volumes of injected vapor are needed because of the relatively low molecular weight and low heat capacity of hydrocarbon vapors. Condensation and separation of injected heating vapor and recovered lubricating components are somewhat expensive.
We wanted to vaporize the valuable lube components in ULO without unduly damaging the lube containing vapor product or fouling the plant heaters. We realized that there was a way to overcome the deficiencies of the prior art process by doing something akin to early treatment of FCC feed. In the FCC process, refiners charge a selected distilled fraction to the cracking unit. A distilled feed is used because distillation leaves behind unwanted metal species which are a poison to the cracking catalyst. Our approach was something like an island hopping campaign in war. We did not care about eliminating the enemy, namely the additive package, if we could get around it.
We found in the effluent of a used lube oil refining process the ideal material to heat the incoming used lube oil. The recovered distilled lubricant boiling range material was essentially a metal and contaminant free material. This material could be safely heated without additive fouling of metal surfaces to produce a super-heated fluid which could be mixed with and vaporize a ULO feed. It may seem counter-intuitive, to use a clean, distilled material that was the desired product of the process, to send this product to a heater at temperatures which could in time thermally crack the product, and then mix this clean superheated product completely with a highly contaminated feed. Refiners usually want to recover a valuable product and send it to storage for sale, not heat it to a temperature which could in time coke it, and then mix it with a highly contaminated feed stock.
The key feature was heating the ULO with a clean, distilled material—a portion of the lubricant boiling range product recovered from the used lube oil fractionator. This material was essentially free of metals and could safely be heated in a heat exchanger or fired heater without fear of fouling. This distilled material, a lubricant boiling range fraction, was always available downstream of the vaporizing portion of the plant. It has a high boiling point, essentially the same as the boiling point of the lubricating oil components of the ULO feed to the process. This vaporized lube fraction could be readily condensed at high temperature. It also, by virtue of its high molecular weight, could carry a lot of energy with it when heated in a furnace or heat exchanger so that undue volumetric amounts were not required to achieve the desired amount of direct contact heating of ULO feed.
We realized that the high grade energy in hot, vaporized lubricant boiling range hydrocarbon vapor recovered from the ULO, could be used to effectively and gently preheat incoming feed for energy savings. The resulting condensed lube fraction could also be split into a net product fraction removed from the process and a recycle fraction sent through a fired heater, heat exchanger or other heating means to generate the hot vapor needed to vaporize the ULO feed. The relatively hot product liquid fraction could be used for a measure of preheating of ULO to facilitate dehydration.
The use of a vaporized lubricant boiling range hydrocarbon simplified the process flow and gave the option to achieve significant improvement in thermal efficiency of the process and facilitate plant operation. Lubricant boiling range materials are always available in the feed ULO, so no purchased working fluid is needed save perhaps at initial startup. The vaporized lube fraction condenses readily at elevated temperature, even under vacuum conditions so fin fan coolers or heat exchangers can easily condense these vapors. Preferably much of the energy in the vaporized lube fraction is recovered by heat exchange with ULO feed.
The condensed lubricant boiling range material recovered from the product is a relatively stable material, at least far more stable than the feed ULO. This stable material may tolerate a lot of heating by heat exchange with heavy residue material withdrawn from the product fractionator, by heating in a fired heater, or by heat exchange with some other high temperature heat source. While the paraffinic lubricant boiling range material is subject to thermal cracking, at least the only cracking products will be those typically experienced during mild thermal cracking of relatively pure hydrocarbons, a modest amount of olefins. Such a stream is a valuable product, suitable for use as cracker feed or, depending on thermal severity, suitable for use as a lubricant blending stock. With hydro-processing it can produce a premium blending stock or fuel. Such modestly cracked streams with some olefins but few dienes may be processed easily using conventional refinery technology, whereas severely cracked streams with high diene content require extra and expensive processing to make them stable enough for further processing.